The use of closed recirculating water reservoirs for use by the general public, for example, swimming pools, spas, hot tubs, decorative fountains, cooling towers and the like, has led to a variety of water quality problems. For instance, improper chemical balances in the water can lead to various types of contamination including bacterial and viral contamination.
The use of chemical sanitizers is a fairly standard water sanitation method. Addition of so-called halogen donor compounds, such as chlorine or bromine are effective sanitizers so long as they are maintained at well defined and constantly controlled concentration levels in the water. It is important that the concentration of these chemical sanitizers is not allowed to become too high which may cause irritation to the users and damage to the water system. Insufficient sanitizers result in a contaminated condition.
The difficulties in maintaining a proper balance of sanitizers may arise from numerous load factors that are difficult, if not impossible, to predict. For instance, in a pool the load factor is typically caused by varying numbers of users. In hot tubs the use of air jets and high water temperatures tend to destroy or remove the sanitizer from the water. Cooling towers are subject to environmental conditions, such as fluctuations in temperature. Indoor decorative fountains may be affected by the air quality in the building, while the fountain water can also affect the air in the building.
Various testing devices exist for determining the chemical balance of the water of pools, spas and the like, for example, colormetric chemical test kits are available that utilize liquid droplets, test strips or tablets which dissolve in the water to indicate a particular level or concentration of sanitizing agents. By removing a test sample of water, for example via a scoop or cup, a seemingly representative sample is deemed to have been taken. A staining agent is then added by means such as an eye dropper or the like. The degree of staining relates to the amount of sanitizer in the water. The amount of sanitizer present is determined by visually comparing the degree of coloring of the test sample against a test scale previously formulated. Further complicating the task of maintaining sanitary conditions in such bodies of water is the fact that studies now indicate there is little correlation between the free halogen, e.g. chlorine, residual readings which are normally used to monitor such bodies of water and the actual bacteriological quality of the reservoirs themselves. Pool and spa maintenance officials have long gone under the assumption that maintaining a free chlorine residual of two milligrams per liter or two parts per million will insure a safe water condition. Thus, the parts per million reading which is determined via the stain comparison, is actually a reflection of the sum of the free chlorine and combined chlorine compounds such as chloramine which are present in the water. These combined chlorine derivatives do not protect from bacteria and/or viral contamination. Additionally, since organic and chemical loading drastically reduce the ability of free chlorine to overcome bacteria, the available free chlorine test kits are of questionable value unless the exact levels of organic contaminants and the particular pH of the water being tested is known.
U.S. Pat. No. 4,752,740 suggests the use of monitoring the oxidation-reduction potential (ORP) as a method of measuring the sanitization levels of water. ORP defines the potential of a sanitizer such as chlorine, bromine or ozone to react with various contaminants. These compounds are known as oxidizers and have the property of "burning off" impurities in the water, for example, body wastes, algae and bacteria. The use of an ORP sensor allows the pool maintenance engineer to measure the potential generated by the active form of the sanitizer and not the inactive forms such as the combined chlorine derivatives. Additionally, ORP monitoring has an advantage in that it is an ongoing electronic process requiring no test chemicals or agents and monitoring of sanitation levels is constantly performed as opposed to being performed on some predetermined schedule basis. Since the potential for disease transmission due to organic loading is far more significant in public spas and pools, use of ORP measurement could be of great benefit in reducing the risk of contamination and disease transmission.
In accordance with standards set forth by the World Health Organization in 1972, maintenance of an ORP level of 650 millivolts is deemed to result in a water supply that is disinfected and in which viral inactivation is virtually instantaneous.
Chlorine is the most widely used oxidizer in the aquatic industry, the primary use being for sanitation of the water in pools and spas. Chlorine, being an oxidizer, is also involved in oxidation reactions with various organics, as well as inorganic and organic nitrogen based substances such as urea, uric acid, amino acids, etc. One of the drawbacks of chlorine is the production of chlorinated byproducts resulting from incomplete oxidation. These byproducts are often volatile and produce undesirable side effects such as irritation of the eyes, sinuses, skin, foul smelling air, and corrosion of air handling equipment.
The health department generally regulate the concentration of Free (HOCL & OCL) chlorine in the water. In some locations, sufficient HOCL is not available to maintain a sufficient rate of oxidation of the demand being contributed to the water. This allows for the accumulation of these undesirable substances. Substances which oxidize following substoichiometric oxidation react with the chlorine producing substoichiometric and/or stoichiometric compounds. Further oxidation with HOCL eventually leads to increased concentration of substances that follow stoichiometric oxidation, such as monochloramines. If enough HOCL is not maintained to meet the stoichiometric ratios needed to drive oxidation of the chloramines, no demand on the HOCL is experienced. However, when the chlorine donor(s) are controlled using ORP control with an optimized ORP setting of between 780-800 mV, the buffering effect chloramines place on the ORP becomes a significant factor. The buffering effect provided by the chloramines reduces the impact on ORP provided by the addition of more chlorine donor(s). The controller feeds more chlorine donor(s) to achieve the optimized ORP. This often leads to levels of Free Chlorine which exceed local maximum limits. In order to meet the maximum limits of free chlorine, the ORP is reduced so as to not exceed the established limit. This allows for the volatile chlorinated compounds to accumulate, thereby increasing the partial pressure which promotes fouling of the air.
Numerous attempts have been made at addressing this problem. "Shocking" of the pool water requires dosing the water with stoichiometric concentrations of chlorine to oxidize the substances. One problem with this method is that there cannot be any bathers present due to the excessive concentrations of chlorine required to meet the stoichiometric levels needed when said undesirable substances have been allowed to accumulate. Another issue is this method is generally applied after the symptoms have appeared, i.e. high combined chlorine, foul odors, etc. In many cases this method fails to rid the water and air of these substances since the concentration of chlorine required is at best a rough estimate (incorporates measuring the combine chlorine in the water). Measuring the concentration of combined chlorine in the water does not take into consideration the accumulated demand that is non-aqueous, e.g. that accumulated on the filter media, walls of the pools, etc. As the chlorine levels rise, some of the accumulated demand is liberated. This gives the appearance that the system had not been driving breakpoint when indeed it probably did for awhile. The fact that the free chlorine levels drop considerably, and the combined chlorine level still appears, is an indication the HOCL must have oxidized the combined chlorine and/or accumulated demand, thereby providing a source of readily available oxidizable substances not originally detected in the water. When the free chlorine levels rise, they oxidize substances in the filters and the remaining system. This releases more substances into the water which were not accounted for, the stoichiometric ratio of HOCL is overtaken, and breakpoint is not achieved.
Ozone has been used as a side stream treatment to destroy these undesirable substances. While effective, ozone cannot be applied to the bulk water of the pool where the contaminants are being added. Also, since ozone cannot be used as a stand-alone treatment since it cannot maintain a residual in the water, chlorine or bromine is used as the primary sanitizer. Besides being expensive and often requiring extensive deozonation equipment, e.g. such as activated carbon, ozone destroys chlorine by attacking the hypochlorite ions, thereby further increasing operational and maintenance cost.
Bromine is sometimes used in place of chlorine because of the belief it does not produce the air fouling byproducts produced by chlorine. However, while bromamines are not as volatile as chloramines, they do possess an odor and irritate the eyes. Bromine also requires an oxidizer such as chlorine or ozone to activate the bromide ion. Operating costs tend to be high and it is often difficult to maintain water quality since no easy methods are available for differentiating between free or combined bromine. Also, hydantoin, an additive commonly used to pelletize the bromine chlorine combination, reduces the oxidizing power of the bromine as the hydantoin accumulates in the water. This makes it more difficult to reduce the accumulation of undesirable brominated compounds.
Non-chlorine shock treatments incorporating peroxygen compounds, e.g. potassium monopersulfate (MPS) have been sold under the brand name OXY-BRITE for addressing the chloramine issue. Despite the application of this product following manufacturer's guidelines, many pools continue to experience chronic air and water quality problems. The method of shock feeding is a means of addressing the symptoms resulting after the problem makes them apparent, e.g. high chlorine concentration and foul odors. MPS is approved for use as a shock treatment while bathers are present. However, when applied to systems using chlorine donor(s) which are fed using ORP control, the system experiences undesirable side effects from shock feeding MPS. The addition of MPS increases the ORP of the chlorine donor(s) system. When MPS is added, the ORP of the system rises above that provided by the chlorine donor(s) As long as the ORP value remains above the set point established for the chlorine donor(s) system, no chlorine donor is fed. Since many of the contaminants entering the water do not react directly with MPS without first being oxidized by the chlorine donor(s), these substances further accumulate, thereby compounding the problem.